Civil Engineering Reference
In-Depth Information
concrete. The final level of the crushed rock will be determined by the specified thickness
of the slab and the variance between the surface of the slab and the surrounding earth.
Refer to the engineers' details and the approved plans to establish the required slab
thickness. For residential construction it is normally 100 mm excluding the thickness of
the beams.
Figure 3.11
Compacted crushed rock and formwork for a concrete slab
Formwork for the concrete slab
Once the crushed rock is in place, formwork is installed around the perimeter of the
house to contain and set the height of the concrete slab. The top edge of the formwork
will be set to match the required overall height of the finished concrete slab. A laser level
is the simplest method of establishing this height, although a dumpy level will do just as
well. You could use a water level, however this would be much more time consuming. A
water level is a simple device comprised of a length of clear plastic tube almost full of
water with the ends sealed closed. To use a water level to establish the required height of
concrete boxing, stumps or anything else, hold the two ends of the tube together so that
there is air at the end of each tube. Secure one end of the tube with the water line at the
required height in the tube. By stretching the other end of the tube to another position,
you will ascertain the level position by recording the level of water in the tube. Water
levels are simple to make, accurate and very inexpensive. If you opt to make your own, it
is best to colour the water with food colouring or similar, as this makes it much easier to
see the water level.
Following the installation of the perimeter boxing or formwork, builder's plastic or
other approved materials is laid in the building envelope to form a moisture barrier
between the earth and concrete. The plastic is laid into the trenches and over the top edge
of the boxing. All joints in the plastic are to be sealed with tape (see Figure 3.12).
